Sound recording with supersonic modulator



Feb. 29, 1944. F. P. HERRNFELD 2,343,182

SOUND RECORDING WITH SUPERSONIC MODULATOR Filed D66. 17, 1942 |H E w 3INVENTOR f. I? HERRNFELD ATTORNEY 23, 1942, to G. W. Willard.

' to O. B. Blackwell and Patented Feb. 1944 UNITED s'rArEs PATENT OFFICESOUND RECOBDIN G WITH S UPERSONIC MODULATOR Frank 1?. Herrnield, LosAngeies, Calif., assiznor to Western Electric Company, New York, N. Y.,a corporation of Incorporated, New York Application December 17, 1942,Serial No. 469,371

6 Claims. (C1. 179-1003) through link circuit 3 to primary 4 oftransformer Tl, the secondary winding of which is divided into twoparts, Si and S2, joined bybypass condensers Cl and C2 in series, thejunction of these condensers being connected to ground.

Terminals 5 and 6 of the windings SI and S2 are connected respectivelyto grids 1 and 8 of vacuum quency is low in level or altogether absent.A

preferred embodiment of the invention is described whereby noiselessphotographic sound records of the variable. density type may be obtainedin a recording system in which the modulator is'a supersonic valve suchas that disclosed in United States Patent 2,287,587, granted June Theuse in photographic sound recording of a supersonic valve of thisgeneral description is described in United States Patent 2,084,201,granted June 15, 1937, to A. Karolua However, the disclosures of theprior'art show no method or circuit means whereby the advantages ofnoise reduction now universally demanded of sound records can be added.c

It is, therefore, an object of the present invention to provide meansfor photographic sound recording with noise reduction using a supersonicmodulator. Reduction of noise in carrier transmission circuits duringintervals of low or absent audio signals is taught. for example, inUnited States Patent 1,681,972, granted'August 28, 1928,

H. A. .Aflel. For the present purpose, however, the greatest'sixnplicityis desired in recording circuits and it is another object of theinvention to provide a novel carrier transmission system of controlledcarrier amplitude characterized by a minimum complexity of apparatus. r

A feature of the invention is the provision of a pair of vacuum tubes bywhich is effected the final step of amplification of both the carrierfrequency and the modulating signal frequency. These tubes are soarranged as to-p'ossess a net conductance varying with the level of themodulating signalbetween the maximum conductance of either tube aloneand a minimum determined by the adjustment of the noise reductioncontrol;

Other objects and features of the invention will be apparent from thefollowing description when 1 read with reference to the accompanyingdrawing in which the single figure schematically represents a circuitadaptedto achieve the objects above declared. I

In the figure, i is an oscillator tuned to a carrier frequency of, say,8 megacycles per second.

Tuning coil 2 of this oscillator is linked as desired tubes Vi and V2which are represented in the figthe as triodes but may, of course, bepentodes if desired. As, indicated, link circuit 3 is adjustably linkedat one end to oscillator tuning coil 2 and at the other end to primary 4of transformer Ti. The secondary circuits SI and S2 of transformer Tlare provided with trimmer condensers 9 and I0 whereby the transformeroutput circuits are accurately tuned to the carrier frequency generatedby oscillator I.

Cathodes H and I2 of tubes Vi and V2, respectively, are maintained atthe desired positive potentials with respect to grids 1 and 8, cathode Hby a tap on potentiometer P2, cathode I2 by a tap on potentiometer P3.These potentiometers, in parallel, are in series with resistance 20between,

the positive terminal of battery i3 and ground.

By-pass condensers l4 and iii are provided in shunt between ground andcathodes Ii and i2, respectively. It is to be understood that thecathodes named are heated by the usual power supply, not shown.

inductance l6 and condenser ll constitute a filter for the anode currentsupply of'tubes VI and V2 which is obtained from battery l3 throughinductances l6 and LI, the purpose of the latter inductance being statedbelow; Anodes l8 and I 9 of tubes VI and V2 are joined together and tothe terminal of inductance Ll remote from battery I 3.

It will at once be observed that the grid-cathode circuits of tubes VIand V2 are in push-pull connection while the corresponding anodecircuits are in parallel. Therefore; if'the conductances of tubes VIand'V2 are equal, no carrier voltage will appear between ground and thejunction of anodes IB and I9 whatever the input carrier voltagetransmitted from oscillator I to the gridcathode circuits of tubes VIand V2. The conductances of these tubes are individually determined byadjustment of potentiometers P2 and P3.

The advantage in efliciency of operating a vacuum tube as a class Bamplifier is well known and it is here preferred so to adjust P3 thattube V2 is just at cut-oil when there is no carrier voltage on grid 8.P2 may then be adjusted so that a push-pull alternating voltage on gridsI and 8 results in no alternating current in the I output circuit or P2may be adjusted to provide a still greater negative bias of grid 1 inwhich case there is a net conductance for tubes VI and V2 over thehalf-cycle of carrier voltage from oscillator I which V2 is permitted topass. The value of this net conductance may be varied by suitablysetting a tap on potentiometer P2 so that the opposition of tube VI totube V2 is complete to any desired degree.

With the tap on P2 so placed that no conductance results in VI, thecircuit functions as a class B amplifier of the carrier voltage receivedon grid 8. If a modulating signal voltage is now impressed across theterminals of resistance M, in series between ground and secondarywindingS2, output terminals X and .Y are capable of transmitting a modulatedcarrier wave to any I desired transmission line or terminal apparatus;

for example, XYmay lead to the crystal 22 of a supersonic valve 23 andthe modulated carriervoltage so impressed on crystal 22 may be used to.modulate the light from source 24 transmitted through valve 23 to film25 asdescribed in the prior art. Film 25 is driven by, conventionalmeans not shown.

'Ii,'now in addition to the modulating signal voltage across resistor2|, it is arranged to provide a control of the net conductance of tubes-VI and V2 in accordance with the envelope of the modulating signal, onemay so arrange matters that the'net carrier voltage appearing acrossterminals X and 'Y is at all times only that which can be fullymodulated by the audio frequency, thereby making possible a noiselessrecording on film 25 of the variable density type.

Before providing audio signal and the noise control it is necessary totune the tank circuit composed of inductance LI and condenser C3 latorI.

the conductance of tube VI. The portion of PI included between tap 30and ground is in series between ground and secondary winding SI oftransformer TI, and the indicated poling of rectifier 28 and connectionof circuit B is such that as the level of the audio signal in circuit Avaries, the conductance of tube VI is correspondingly varied, therebyproviding a net conductance for tubes VI and V2 which varies with thelevel of the audio signal across resistor 2| which modulates the carriervoltage derived from oscil- It may be preferred, as insurance of greaterfreedom from distortion in circuit A, to provide for each of circuits Aand B a separate amplifier succeeding amplifier 21.

The adjustment of the circuit prior to recording the-audio signalreceived by microphone 26 is as follows: Oscillator I is energized andtuned to the frequency of crystal 22 which is itself for this adjustmentreplaced by a dummy load, tube VI being removed from the circuit. V2 isadjusted for class 3 operation. The output circuits of transformer TIand circuit LI-C3 are tuned for maximum carrier output and link circuit3 is adjusted to provide a carrier voltage on voltmeter VM suflicing topermit full modulation of the light transmitted by supersonic valve 23.I Thereafter crystal 22 and tube VI are replaced and P2. is so adjustedthat VI is just at cut-oil with the! carriervoltage applied.

Switches DI and D2 are provided so that in preliminary adjustment eithercircuit A or circuit B may, as desired, be replaced by a resistor 3| or32, respectively, without disturbing the adjustment of the companioncircuit. If these,

switches are in position as indicated in the figure and the operationsjust described "have been performed, conditions are such that the wholeunit to the carrierv frequency derived from oscillator- I and byadjustment of the coupling of link cir-. cuit,3 to provide acrossterminals X and! a carrier voltage (tube VI being fully blocked for thispurpose) of magnitude required ,to drive the" crystal 22. A vacuum tubevoltmeter VM is provided to measure the voltage between the terminals Xand Y. Tank circuit LI-C3' serves to provide a sine wave of carriervoltage derived from the class B output of tube VI. A crystal moduelator'of the type here considered is sufllciently highly tuned so thatnodetuning of oscillator I is to be feared as the conductances of tubesVI and V2 are varied. It will be noted that secondary SI of transformerTI- is conductively connected to ground through a portion ofpotentiometer PI in the upper part of the figure. A steady or slowlyvary- .ing voltage across PI determines the control of the netconductance of tubes VI and V2 as this dependsupon the grid bias of tubeVI.

For the carrier transmission circuit so far described, a modulating andnoise controlling cirnoise reduction may now be-made on film 25.x

cuit is provided comprising microphone 26 sucaudio voltage is impressedacross resistor 2|. simultaneously, a portion of the output of amplifier21 is rectified byv a full wave rectifier and'supplied through filter 29so that a rectified voltage, the envelope of the audio signal, is 'var-4 operates as a class B modulator wherein an input sound signal receivedby microphone 26 modulates the carrier voltage derived from oscillatorI. The output of rectifier 28 and filter 29 is so poled in conjunctionwith the initial adjustment of potentiometer P2 that tube VI is notconductive no matter how high the audio input signal may be. A record ofthe microphone signal without The level of this signal is read on volumeindicator VI.

To apply noise reduction, the audio input is removed by suitablyoperating switch DI and potentiometer P2 is soadjustedthat theconductance of VI efiects partial cancellation of tube V2. Voltmeter VMreads, say EI when VI is permitted a partial conductance and E2 when VIis blocked, so that noise reduction in decibels may be stated as I I [N0i a--j The setting of tap 30 on potentiometer PI is ,a

margin setting rovidedfor the accommodation of abrupt increasesin audiosignal level. Y

In the foregoing description there has been set forth the application ofthe present invention tothe photographic recording ofsound, but it willbe clear that the invention is in no way limited to iably selectedby tapso of potentiometer PI and applied through. circuit B to grid Jtacontrol such application and may be used wherever carrier transmissionof low frequency signals is f required with or without the advantage'ofsuppressing to a desired extent the carrier frequency during periods.when theumidulating'signal is of low level or absent.

What is claimed is: 4 l. Ina carrier transmission circuit. means for2,843,182 generating a voltage of carrier frequency, meansforsimultaneously and differentially amplifying said carrier voltage inopposite phases in a pair of transmission paths, means for controllingthe relative amplification in said paths, .means for uniting theopposite phases of said carrier voltage as differentially amplified,means for generating a voltage of signal frequency, means for rectifyinga portion of said signal voltage, and means for simultaneouslycontrolling in accordance with said rectified portion the amplificationin one only of said paths and modulating said carrier voltage in theother only of said paths by said signal voltage.

2. In a carrier frequency transmission circuit,

.a carrier frequency oscillator, an output circuit for said oscillator,power supply for said oscillator, a pair of vacuum tubes provided eachat least with a control grid, a cathode and an anode, power supply forsaid tubes, a terminal connected to both of said anodes, a groundedterminal to which said cathodes are individually connected throughadjustable resistances relatively adjusted to provide a net conductancefor said vacuum tubes, individual input circuits between said controlgrids and said grounded terminal comprising each a resistor in serieswith inductive coupling to the output circuit of said oscillator, saidinput circuits being connected for push-pull coupling of said controlgrids to said output circuit, a source of audio frequency current, meansfor impressing across the resistor in one of said input circuits avoltage proportional to the instantaneous magnitude of said audiocurrent, means for rectifying a portion of said audio current, and meansfor-impressing across the resistor in the other of said input circuits avoltage proportional to the magnitude of said rectified current.

3 A sound recording circuit including in combination a supersonic lightvalve having a pair of terminals, a source supplying supersonic voltage-to' said valve comprising a supersonic oscillator, a pair of vacuumtubes provided each at least with, a cathode, a control grid and ananode, said cathodes being individually connected to ground throughvariable resistances, said control grids being inductively'coupled mpush-pull to said oscillator and said anodes being connected togetherand to the ungrounded terminal of said valve, a source of audiofrequency current, means for supplying to one of said control grids avoltage proportional to the instantaneous value of said current, meansfor providing a voltage of supersonic frequency, power supply for saidoscillator, a pair of amplifying tubes provided each at least with acontrol grid, a cathode and an anode, power supply for said tubes, acommon connection of said anodes to an output terminal, adjustablebiasing resisttors individually connecting said cathodes to a commongrounded output terminal, said biasing resistors being separatelyadjusted to provide unequal conductances for said tubes, input circuitsindividual to said tubes including each a series resistance connectedtosaid grounded terminal, inductive coupling between said oscillator andsaid input circuits in push-pull connection, a source of voltage ofaudio frequency, means for applying said audio frequency voltage acrossthe series resistance in one of said input circuits,

- means for rectifyingca portion of said audio frequency voltage, andmeans for applying said rectified voltage across the series resistancein the other of said input circuits whereby there is produced betweensaid output terminals a supersonic voltage modulated in accordance withthe instantaneous value of said audio frequency voltage, and of averageamplitude controlled by said rectified voltage;

5. In a signaling system in which a supersonic frequency is modulated bya signal of audio frequency, the method which consists in generating avoltage of supersonic frequency, amplifying said supersonic voltagedifferentially and oppositely in phase in a pair of transmission paths,combining'said paths to produce a net amplified supersonic voltage,generating 2, voltage of audio frequency,.introducing said audio voltagein one only of said paths, rectifying a portion of said audio voltage,introducing said rectified voltage in the other only of said paths, andsimultaneously controlling the level of said net supersonic voltage inaccordance with the envelope of said rectified voltage] and modulatingsaid net supersonic voltage by said audio voltrectifying a portion of,said current, and means for supplying to the other of said control gridsa voltage proportional to said rectified current whereby the supersonicvoltage-applied to said valve is varied in amplitude in accordance withthe envelope of said rectified current.

4. In a sound circuit in combination, an oscillator having an outputcircuit age.

6. In a carrier signaling system in which a carrier frequency ismodulated by signal frequency, the method which consists in generating avoltage of said carrier frequency, amplifying said carrier voltagedifferentially and in opposite phase in a pair of transmission paths,combining said paths to produce a net amplified supersonic voltage,generating a signal voltage,

introducing said signal voltage in one only of said paths, rectifying aportion of said signal voltage, introducing said rectified voltage inthe other only of said paths, and simultaneously controlling the levelof said not carrier voltage in accordance with the envelope of saidrectified portion and modulating said net carrier voltage by said signalenvelope.

FRANK P. HERRNFELD.

